The Prolactin hormone and paternal care in primates
Introduction
Prolactin is a peptide neurohormone secreted by the anterior pituitary gland and regulated by the hypothalamus. Even though it has been reported to have around 85 different biological functions it is best known for its effect on lactation and maternal behavior (Kelly et al, 1998). For this reason it is generally considered a female hormone, but it is also present in the male gender, where it naturally does not have any affect on lactation. Despite the absence of lactation in the male, studies have shown that prolactin might actually have an influence on paternal care for the infants in various species. Paternal care will in this essay be considerd as a suite of behaviors performed by a mature male (the putative/social father of the immature young), which would not be perfomed in the absence of the young (Valeggia et al, 2009). The intention of this essay is to explore more deeply the effect prolactin has on the expression of paternal care, focusing on our own animal order, the primates.
The role of prolactin in male primates
Prolactin and paternal behavior
The influence prolactin has on paternal care have been intensely studied in birds and fish, and suggest that there is a significant relationship between prolactin plasma concentrations and paternal care in these species. This has lead scientists to believe that prolactin is important in the expression of paternal care. (Schradin and Anzenberger, 1999), (Ziegler et al, 2009). Fewer studies have been carried out concerning primates but as more exploration of the domain is conducted there appear to be a connection between prolactin and paternal behaviour in several primate species as well (Zagoory-Sharon et al, 2010). It has not been proved that prolactin and paternal care has a causal relationship (Valeggia et al, 2009) but a number of studies show that the prolactin plasma level is significantly higher in fathers than in non-fathers. Additionally, it is know that horomones can have a direct effect on behavior when they implement their action on the Central Nervous System (Schradin and Anzenberger, 1999). Even though prolactin is a hormone built up by approximately 200 amino acids, and there is some debate to whether or not it can cross the blood-brain barrier, the coroid plexus does have prolactin receptors and is believed to play a considerable role in the movement of prolactin across the barrier and into the brain (Schradin and Anzenberger, 1999). These factors may suggest that prolactin has a designated role in paternal behaviour.
Experienced versus inexperienced fathers
Indeed, it has been established that in some primate species the level of circulating prolactin is different in males that are fathers and males who are not fathers (Zagoory-Sharon et al, 2010). Several investigations show that prolactin levels are increased pre-partum in fathers, as well as in the pregnant female, for instance in the cotton-top tamarine (Saguinus oedipus) and the common mormoset (Callithrix jacchus) (Ziegler et al, 2009), (Zagoory-Sharon et al, 2010). Another common observation is that experienced fathers, usually having fathered three or more offspring, have a higher level of prolactin than inexperienced fathers and that they also show a higher amount of parternal care and react in a more positive way to the infant (Ziegler et al, 2009), (Zagoory-Sharon et al, 2010). It has been discovered that when the father holds for the first time or he hears the cry of his first child, the prolactin concentration would decrease compared to the pre-partum period, whereas it increases if the father is not a first-time father. These observations has lead to the belief that some particular hormonal changes happens when the male becomes a father for the first time (Schradin et al, 2003), (Zagoory-Sharon et al, 2010). Also, according to Gordon et al (2010), there seems to be a base-line level of prolactin which implement a trait-like characteristic of the individual (Zagoory-Sharon et al, 2010). when he has his first child. These observations coincide with care-giving being one of the main factors for infant survival, and in that way ensuring the transfer of the fathers genes to the new generations (Schradin and Anzenberger, 1999).
Hormone regulation
Prolactin fluctuations
Prolactin level in human fathers are higher before birth than after, and demonstrate a quite stable hormone concentration for the first six months of the infants life (Zagoory-Sharon et al, 2010), whereas in Cottontop tamarins the prolactin level in the father increase around two weeks prepartum and reach a maximum level two weeks after birth (Schradin and Anzenberger, 1999), (Ziegler et al, 2008). Although there is some variation among these species, it has been clearly demonstrated that the presence of a pregnant mate, and later the infants presence, could be associated with elevated levels of prolactin in the male (Ziegler et al, 2008). The Titi monkey (Callicebus cupreus), Common marmoset and Goeldis monkey (Callimico goeldii) all have different commencement of the increased prolactin levels. In some the level was always high, whereas in others the level was raised in the period before the offspring was born. However, in all three cases, the level of prolactin was the highest 1-3 weeks after the birth of the offspring (Schradin et al, 2003). These studies demonstrate that prolactin production is not only caused by infant stimuli and might mostly be important for the onset of paternal care and not so much for maintainig it (Schradin and Anzenberger, 1999). In female rats it has been discovered that prolactin is necessary for the mother's caring for the young but that steroid hormones are essential in order to induce the effect prolactin has on the maternal behaviour (Schradin and Anzenberger, 1999). Moreover, this seems to also be the case with onset of paternal care and it is possible that steroids induce the expression of prolactin receptors (Schradin and Anzenberger, 1999). Prolactin and testosterone usually have a negative relationship toward one another; when one decrease the other increases and vice versa. This relationship is reduced both during elevated and decreased prolactin levels. Estradiol also has a negativ relationship with prolactin, but it, unlike the relationship with testosterone, are mentained with both decreased and increased prolactin levels; decreased prolactin levels resulted in an elevated estradiol level and vice versa. (Ziegler et al, 2009).
Interspecies differences
Paternal care in primates is clearly varied among species (Valeggia et al, 2009) but in human and some monkey species, such as tamarins, marmosets, titi monkey and owl monkey there is a considerable level of paternal care (Valeggia et al, 2009), (Schradin and Anzenberger, 1999). It is direct and lasts for a considerable amount of time and is observable at different circumstances (Valeggia et al, 2009). In the marmosets and tamarins it is also obligate (Schradin and Anzenberger, 1999). In different species of the New world monkeys, the onset of the paternal behavoir occured at variable times in the infants life in different species of the new world monkeys . In two species , the father started caring for the infant within the first day, while in one the start for paternal care was 3 weeks after the birth of the offspring (Schradin et al, 2003). The levels of prolactin were most similar in those monkeys having a similar way of interacting with the infant. In the monkey species where the primary care is given by the father, the level of prolactin were the highest (Schradin et al, 2003). In human fathers, the males that were most influenced by signals given by the infant presented higher proalctin levels than the more passive fathers. The prolactin hormone in the father was associated with his promting and reinforcing of the exploratory behaviour and play of the child (Zagoory-Sharon et al, 2010).
Family structure
In all of the New world monkeys studied in this case, the families were built up by not only the mating pair, but also by adult and subadult offspring, called helpers. The level of circulating prolactin were not as high in the helpers even though they took part in the care of the young (Schradin et al, 2003). There was a significant difference in the level of prolactin in general between the father of the infant and their adult sons that served as helpers. Also the same male showed differences from the time he himself was a helper until the time he was the father. In the second case the level of prolactin was higher. In one species (C. Jacchus) fathers had higher levels of prolactin during times without preambulatory infants than their adult sons did (Schradin et al, 2003). Cotton-top tamarines also show increased prolactin levels during midpoint of their mate´s pregnancy, but this conserned only experienced fathers and not first time fathers. The age of the male showed importance only when compared with the adult sons. When the two groups were considered alone, age had no significant importance. A young father showed higher levels of prolactin than an older male who only surved as a helper (Schradin et al, 2003). Another interesting fact concerning the cotton-top tamarines is that the timing of the prolactin peak as averaged by month, correlates exactly with the number of infants surviving from the previous birth (Ziegler et al, 2008).
A unique experiment
Studies conserning the Common marmoset has given some very interesting results. In an investigation in which 15 experienced fathers where chosen and studied during three pregnancy periods, one with normal prolactin levels, one with decreased and the last one with elevated prolactin levels. The decreased prolactin gave a decrease in the fathers responsiveness toward the young, as one could expect, but the elevated dose also gave the same result. Another important discovery that this study showed was prolactins effect on weight loss. Normally fathers of the common marmoset loses a lot of weight during infant caring, because of the extra energy recuired for carrying, feeding etc. With the decreased treatment the faters lost even more weight than normal, but higher levels resulted in less or even no amont of weight loss (Ziegler et al, 2009).
Summary and conclusion
Untill now there is no absolute proff that prolactin and paternal care have a direct relationship, but much reasearch are suggesting that they do. One major fact pointing towards this is that prolactin plasma level is significantly higher in fathers than in non-fathers. The cause of this increase in prolactin levels conserning fathers have both been suggested to be caused by the precens of infants and an experienced mother, but neither has been succesfully confirmed. There are many interspecial diffrences among the primat order conserning the supposed relationship of paternal care and prolactin, both concerning the timing of increased prolactin levels and its effect. One thing that so far has been common in all studied species is the higher level of prolactin in experienced fathers compared to new one. Even thought prolactin have not been proven to have a direct relationship with paternal care, it is very likely that it will have a indirect effect, since it generally have more than 85 different biolobical functions and have a close relationship with other steroidal hormones and has been shown to reduce weight loss. There is certainly a relationship between paternal care and prolactin levels in primates, but in what form more research is needed to answer that question. Especially more species specific studies, since there are many interspecies differences.
References
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